1. Field of the Invention
The present invention relates generally to metalinsulator-metal (MIM) capacitors for monolithic microwave integrated circuit (MMIC) applications and, more particularly, relates to a capacitor-around-via structure for MMICs.
2. Brief Description of the Relevant Art
The trend in microwave-frequency electronic systems is toward increasing integration, reliability, and lower cost in large-volume production. Typical microwave-frequency applications include satellite communication systems, phasedarray radars, electronic warfare systems, and the like.
Since the early 1980s, the widespread availability of monolithic microwave integrated circuits (MMICs) has spearheaded the drive toward greater integration and reliability. MMIC technology entails a multi-level process approach, characterized by all active and passive circuit elements and their interconnections being formed into the bulk or onto the surface of a semi-insulating substrate (e.g., GaAs). The circuit elements and interconnections are fabricated by utilizing some deposition scheme such as epitaxy, ion implantation, sputtering, evaporation, diffusion, or a combination of these processes.
The passive circuit elements for monolithic microwave circuits include distributed elements (transmission lines), traditional lumped elements (capacitors, inductors, and resistors), and circuit interconnections (via holes and air bridges). Capacitor structures are used, inter alia, for tuning elements, for interstage isolation, for RF bypass, and for matching applications.
Depending upon the application being considered, MMIC designers may choose between (1) interdigitated capacitors for low capacitance values (less than 1 Pf); and (2) parallel plate, metal-insulator-metal (MIM) capacitors for higher capacitance values (1-20 pF). The MIM capacitor is formed by two metallized areas separated by a dielectric material.
One plate of a MIM capacitor often must be grounded, preferably by a low-inductance connection. State-of-the-art MMIC designs typically employ via holes to make a low-inductance ground connection between a backside ground plane and a frontside, first-level metal layer serving as a capacitor plate. Via-hole technology for MMICs involves etching holes through the GaAs substrate and then metallizing the interior of the via hole and the back surface of the wafer to provide a good connection between the overlying frontside metal and the backside ground plane.
A reliable design for MIM capacitors is important because some MMIC circuits contain more capacitors than any other component. Thus, the capacitor yield could dominate the overall yield of the MMIC. Thus, to preserve their considerable investment in the already-completed frontside fabrication, most foundries take a conservative approach to implementing capacitor-via structures, particularly with respect to backside processing of via holes.
Two primary capacitor-via-hole structure have been realized to date. The earliest embodiment of these is the capacitor-near-via structure, characterized by a rectangular MIM capacitor placed adjacent to a via pad. Shown in FIG. 1, this prior art capacitor-near-via configuration presents no fabrication problems and is compatible with most processes. The MIM parallel-plate capacitor is fabricated adjacently to the via hole, and, typically, the via hole pad is electrically common with the first-level metallization (bottom plate) of the MIM. Unfortunately, the capacitor-near-via structure is wasteful of circuit area and is the source of undesirable parasitic path inductances.
A second, later-developed embodiment is the capacitor-over-via structure, characterized by a MIM capacitor placed directly over the via pad. Shown in FIG. 2, this prior art configuration represents a significant improvement over the capacitor-near-via structure because the capacitor-over-via structure requires less space and minimizes the ever-present parasitic path inductance problem. Unfortunately, this structure is difficult to fabricate and is not compatible with processes employed by most MMIC foundries. Accordingly, the capacitor-over-via structure has not been widely adopted.
Thus, there exists a need in the art of MMIC design for a new MIM capacitor structure that has beneficial properties similar to the capacitor-over-via structure, yet is compatible with processes in most available foundries.